This invention is directed to an electrically conductive silicone gel containing conductive particles of silver, preferably in flake form, and silver coated mica. The invention has particular utility for the electrical interconnection of a plurality of conductors, such as discrete insulated wires, printed circuit board traces, conductors on a flat panel display, or pin grid array.
The prior art is replete with reference to the use of such materials as conductive epoxies and thermoplastic adhesives in devices to establish interconnections for electrical circuitry. For example, conductive epoxies have been employed to mount electrical components on printed circuit boards. Polymeric electrically conductive adhesives have also been employed in tape form to interconnect conductors on a substrate with other components or circuitry. When used to establish such electrical interconnections, these conductive epoxies and conductive adhesives can be deposited on a surface in a conventional manner, such as by screen printing. Then upon the application of heat and pressure or upon the application of pressure, depending upon the precise character of the conductive adhesive, both electrical and structural or mechanical integrity can be established between separate conductors. These conductive epoxies can be formulated to harden to form a mechanical connection, or they can have a tacky, putty-like consistency when cured. Although conductors can be removed from the putty-like material, the tacky material will adhere to the conductors upon removal.
These polymeric interconnection systems generally are intended to replace such conventional interconnections as soldered interconnections. However, these polymeric electrically conductive materials generally form permanent bonds between the conductors in somewhat the same manner as with the use of solder. Although polymeric conductive materials, unlike solder, do permit initial checking for electrical integrity prior to establishment of the final soldered interconnection, in use such conductive polymers establish a permanent interconnection between associated conductors.
U.S. Pat. No. 4,770,641, to Rowlette, represents an early commercial development of an electrically conductive gel. An attribute of such gel is its ability to be disconnected. The gel thereof comprises a polymeric conductor system in which a plurality of conductive particles are dispersed within an insulative gel-like medium. The conductive gel possesses a memory and is conformable to the cavity prior to interconnection of the associated conductors. The gel is not in a liquid state and is sufficiently coherent and viscous that it will not flow from the cavity means under the influence of gravity. The gel will also conform to the interface between the housing cavity and the conductors upon interconnection. Upon disconnection, however, the gel will return substantially to its original shape and remain within the housing cavities. The gel will not adhere to the conductors upon disconnection.
U.S. Pat. No. 4,406,827, to Carim, represents a different approach to an electrical conductive gel. The gel, developed for low resistance contact between a metal electrode and a biological body, comprises an aqueous solution of up to saturated concentrations of ionized salts as the conducting agent, a natural gum capable of crosslinking, and crosslinking material which provides the electrically conductive gel with sufficient internal strength to remain cohesive without reinforcement.
U.S. Pat. No. 4,845,457, to Nakanishi, is directed to a deformable variable resistor which employs a gelled silicone resin. The gel is characterized as having a defined penetration value as a basic member in which electrically conductive fine particles as much as 20 to 50 weight percent are mixed, which basic member is provided with at least a pair of electrodes so that the conductive fine particles contained in the basic member come in contact each other to form a number of electrical passages between the pair of electrodes when the basic member is physically deformed by an external force.
The present invention possesses the attributes of the prior art while avoiding the shortcomings or limitations thereof. The gel according to this invention is non-flowing, self healing, and thermally stable. That is, the silver flake and silver coated mica loaded gel is highly conductive over a range of temperatures. Further, by the unique combination of such silver flakes and silver coated mica the particles remain uniformly dispersed within the gel prior to curing of the gel. This assures a highly and uniformly electrically conductive gel. The various and numerous features of the gel according to this invention will become apparent in the description which follows.